1. Field of the Invention
This invention lies in the field of phase separations of mixtures containing organic compounds, solids, and water. The invention relates in particular to processes involving distillation and membrane separations.
2. Description of the Prior Art
Fluid mixtures containing low boiling organics (LBOs), solids and water are generated in a variety of unit operations throughout the chemical and biotechnology industries. Examples of the industries that have unit operations where such mixtures may be generated are pharmaceuticals and biopharmaceuticals manufacturing, biofuels manufacturing, food, flavor and fragrance industries, intermediate chemicals and petrochemicals manufacturing, petrochemical refining, and natural gas suppliers. Reactors, extractors, precipitators, and crystallizers are examples of unit operations or processing steps that generate such mixtures. The reactors may be catalytic and non-catalytic and, in the catalytic reactors include those using chemical catalysts or biocatalysts for biochemical conversions such as fermentation.
Fermentation-based processes are increasingly used for the production of organics, including for example the production of ethanol and butanol from agro (starch- and sugar-based) feedstocks, cellulosic and lignocellulosic feedstocks; and industrial waste-based feedstocks such as cheese whey. Typically, the feedstocks are first treated to produce a necessary intermediate such as a fermentable carbohydrate (sugar) or bio-based syngas.
The fluid mixture produced by the fermentation of sugar intermediates contains at least one low-boiling organic substance in large concentrations, other organic substances in smaller concentrations, water, suspended solids and dissolved solids. The fermentation process also produces carbon dioxide. The low-boiling organics are produced in large concentrations and are usually the main product of the process and must be recovered and dried. The solids, however, are also a valuable by-product and likewise usually require concentration and drying. Both recovery and drying of the organics and concentration and drying of the solids are energy-intensive operations. With rising energy prices, there is a significant need for the development of novel energy-efficient approaches to achieve the desired separations.
The fermentation process described above is used for producing ethanol from sugar-based feedstocks such as cane molasses and from starch-based feedstocks such as corn. The use of ethanol for fuel blending is rapidly growing worldwide. Fuel-grade ethanol typically containing less than 0.5% water by weight is produced by multiple distillation steps using atmospheric distillation, vacuum distillation, or multi-pressure distillation to concentrate the ethanol to the azeotropic concentration followed by an azeotropic distillation or an adsorption step for recovery and drying of the ethanol. A solids-and-water stream is removed from the stripper column, which is the first distillation column in the distillation train.
In molasses-based plants the solids-and-water stream is usually called “spent wash,” whereas in corn-based plants it is called “whole stillage.” The spent wash and whole stillage streams are treated to recover both the solids and water which can be re-used. The spent wash stream can be used for ferti-irrigation after bio-gas generation, or it may be composted or evaporated. Evaporation is becoming increasing popular as a means of reducing the volume of the spent wash stream to reduce the land requirements for composting. The concentrated stream can also be used in a boiler as fuel, or incinerated or sold as condensed molasses solubles (CMS), which is useful as a cattle feed additive.
A whole stillage stream from a stripper column is typically treated and concentrated to produce either wet distillers grains (WDG), wet distillers grains and solubles (WDGS) containing up to 70% water by weight, distillers dried grains (DDG), or distillers dried grain and solubles (DDGS) containing up to 10% water by weight. The solids-water stream from the first distillation column (stripper) in the recovery of ethanol is typically directed to a centrifuge followed by evaporation for production of WDGS and an optional drying step for production of DDGS. International Patent Application Publication No. WO 2004/088230 A3, entitled “Ethanol Distillation With Distillers Soluble Solids Recovery Apparatus” (Brown, Thermal Kinetics Systems, LLC), publication date Oct. 14, 2004, discloses the conventional process for the production of ethanol with slight modifications.
For the drying of ethanol, the pressure swing adsorption process (PSA) has earned industry-wide acceptance and has virtually replaced azeotropic distillation all over the world due to its reduced energy consumption and its elimination of “entrainer” requirements. The PSA process typically uses two beds of Zeolite A molecular sieve beads in cyclic batch mode. In the adsorption cycle, the azeotropic mixture (95% ethanol-5% water) from the distillation column flows through Bed 1 and anhydrous alcohol is produced. A fraction of the product flows as purge through Bed 2 that is in a regeneration cycle. As a result, the regenerant stream contains about 60-80% alcohol that must be recycled to the rectifier and re-distilled to the azeotropic concentration for recovery.
Vander Griend U.S. Pat. No. 7,297,236, issued Nov. 20, 2007 discloses an integrated process for both organic-water separation and solid-liquid separation using conventional technologies. The process uses an energy-intensive multi-distillation step, combined with a molecular sieve step for ethanol recovery and an energy-intensive evaporation step for solids concentration.
Membrane separations are used in the chemical industry for a broad range of applications. Separations known as “Molecular Dehydration (Organic Dehydration)” and “Solid-Liquid” separations are examples of such applications.
Membrane-based processes for molecular dehydration are highly energy-efficient, environment friendly, easy to integrate into existing plants and to operate, and have reduced maintenance requirements compared to conventional distillation and adsorption-based processes. The separation in a membrane-based process is based on the difference in partial pressure (chemical potential) between the feed side and permeate side of the membrane and not on the relative volatility. The use of entrainer chemicals, therefore, is not required for breaking azeotropes. Moreover, there are no regeneration requirements since the membranes do not get saturated as do adsorbent beads. Membranes thus perform dehydration in a simple, non-cyclic continuous process. Energy efficiency is also achieved in membrane-based solid-liquid separations, since the separation (by concentration of solids) is based on a pressure differential between the feed side and the permeate side, without requiring the evaporation of liquids. Membrane-based systems also offer a smaller footprint than evaporator systems.
Examples of prior art disclosures of membrane-based processes for the treatment of organic-water mixtures and solid-liquid mixtures are Kaschemekat et al. (Kernforschungszentrum Karlsruhe GmbH) U.S. Pat. No. 4,900,402, issued Feb. 13, 1990, and Vane et al. (Membrane Technology and Research Inc.) U.S. Pat. No. 6,755,975, issued Jun. 29, 2004. These patents disclose membrane-distillation and dephlegmation-based processes for the separation of liquids. Both references are silent however on the treatment of solids and both assume that the feed to the distillation/dephlegmation step is solids-free. Dijkstra et al. (GKSS-Forschungszentrum Geesthacht GmbH), in German Patent No. DE 103 33 049 B3, granted Nov. 25, 2004, disclose an energy-efficient membrane-molecular sieve adsorption process integrated with distillation. This reference also ignores the problem of solids separation and concentration, however.
International Patent Application Publication No. WO 2005/113118 A3, entitled “Fuel And By-Products From Fermentation Still Bottoms” (Ahring et al.), publication date Dec. 1, 2005, and its United States counterpart U.S. Pat. No. 7,267,774 B2 (Peyton et al., NouvEau, Inc.), issued Sep. 11, 2007) disclose an energy-efficient membrane-based solid-water separation process for the concentration and treatment of dissolved and suspended solids from the bottoms of distillation columns. This reference is silent on the recovery of organics.
Canadian Patent Application No. CA 2 523 099 to Kaiser et al. (BUSS-SMS-CANZLER GmbH, published Nov. 11, 2004) and its published United States counterpart US 2007/0131533 A1 (Blum et al., published Jun. 14, 2007) address issues related to both ethanol recovery and solids concentration. The membrane in this reference is used for the drying of ethanol, but the reference is silent on the treatment of the bottom stream of the distillation column. The reference does however disclose a solid-liquid membrane-based treatment for processing the feed mixture from the fermentor, which consists of ethanol, water and solids.
Mairal et al. United States Patent Publication No. 2007/0031954 A1, publication date Feb. 8, 2007, also addresses issues related to ethanol recovery and solids concentration in the same application. The membrane process in this reference is used for organic dehydration, and a solid-liquid membrane-based treatment is used for processing a mixture of ethanol, water and solids from the fermentor. The reference is likewise silent on the recovery of ethanol from the discharge stream from the solids removal step.
It is clear from the patent literature that attempts are being made to improve the current processes for the treatment of fluid mixtures containing low-boiling organics, water and solids. None of the patents or patent applications, however, disclose an integrated energy-efficient process that simultaneously provides both organic-water separation and solid-liquid separation. The current invention solves this key problem and provides a novel process and system that separates low-boiling organics, solids and water from a fluid mixture in a highly energy-efficient and cost-effective manner.